The present invention relates to a process for the preparation of linear low density polyethylene. More particularly, the present invention relates to the copolymerization of ethylene and 1-butene using a catalyst system comprising a titanium-containing component and a second component comprising a mixture of a dialkyl aluminum halide and a dialkyl magnesium or alkyl lithium compound.
Linear low density polyethylene, or LLDPE, is a solid branched copolymer of ethylene and 1-butene, wherein the 1-butene content is about 4 to 8% of the total copolymer. This copolymer is conventionally prepared by the Ziegler-catalyzed reaction of essentially pure 1-butene with ethylene in a saturated hydrocarbon solvent.
It is known in the art that extremely reactive Ziegler-type catalysts can be prepared by reacting titanium tetrachloride with ethanol in the presence of a pulverized solid support, such as magnesium chloride. Frequently, esters of aromatic carboxylic acids are included in this preparation. The resulting solid complex is then mixed with a trialkyl aluminum compound, such as tri-isobutyl aluminum, to produce the final catalyst. When used to polymerize alpha olefins, these catalysts provide high yields of stereoregular polymer per part of catalyst. As a result, the catalyst normally does not have to be separated from the polymer to give a final polymer product of specification purity.
Accordingly, U.S. Pat. No. 4,143,223 describes an alpha olefin polymerization or copolymerization process using a catalyst composition comprising a mechanically pulverized solid halogen-containing titanium catalyst component and an organoaluminum compound. The titanium catalyst component is obtained by reacting a titanium compound, such as titanium tetrachloride, with a pulverized solid product derived from a magnesium compound, an organic acid ester and an active hydrogen-containing compound selected from alcohols and phenols.
U.S. Pat. No. 4,107,415 describes an alpha olefin polymerization process using a catalyst system comprising a catalyst-forming component which is an addition and/or substitution product of an electron-doner compound (or Lewis base) and an alkyl aluminum compound, and a supported catalyst-forming component obtained by contacting a titanium compound with a carrier which is a mixture of an active magnesium or manganese dihalide and a solid organic material which is inert to all other catalyst components. The electron-doner compound is typically an organic acid ester and the inert solid organic material is a compound such as durene, anthracene, acenaphthene or diphenyl.
Chemical Abstracts, 96:69614c (1982) describes an alpha olefin polymerization catalyst comprising an organic aluminum compound, such as triethyl aluminum, and a solid component obtained by reacting milled mixtures of magnesium dichloride, aromatic carboxylic acid esters and silicon tetrachloride with titanium tetrahalide in the presence of chlorinated hydrocarbons, such as dichloroethane.
A major drawback of prior art processes for making LLDPE is the fact that conventional Ziegler-type catalysts are highly sensitive to impurities and thus require highly purified alpha olefin monomers for use in copolymerization. This is an important factor to consider when assessing the economics of LLDPE manufacture.